Method of manufacturing amorphous metallic foam

ABSTRACT

Metallic foams comprising high viscosity materials and apparatuses and methods of manufacturing such foams, and more particularly methods for controllably manufacturing metallic foams from bulk-solidifying amorphous alloys are provided.

FIELD OF THE INVENTION

The present invention is directed to amorphous metallic foams and novelmethods of manufacturing amorphous metallic foams; and more particularlyto amorphous metallic foams made from bulk-solidifying amorphous alloysand methods of manufacturing such foams.

BACKGROUND OF THE INVENTION

Metallic foams are known to have interesting combinations of physicalproperties. They offer high stiffness in conjunction with very lowspecific weight, high gas permeability, and a very high energyabsorption ability. Today, these materials are emerging as a newengineering material. Foams can be classified as either open or closedporous. Whereas open foams are mainly used as functional materials suchas gas permeability membranes, closed foams find application asstructural materials such as energy absorbers or light-weight stiffmaterials.

However, the broad-use of metallic foams is hindered by the difficultyin producing uniform and consistent foam structures. Specifically, priormanufacturing methods for producing metallic foams result in anundesirably wide distribution of cell and/or pore sizes, which cannot becontrolled satisfactorily, and as such limits and degrades thefunctional and structural characteristics of the metallic foammaterials.

The conventional production of metallic foamed structures is generallycarried out in the liquid state above the melting temperature of thematerial, though some solid state methods have also been used. Thefoaming of ordinary metals is challenging because a foam is aninherently unstable structure. The reason for the imperfect propertiesof conventional metallic foams comes from the manufacturing processitself. For example, although a pure metal or metal alloy typicallyconsists of a large volume fraction (>50%) of gas bubbles, manufacturingmetallic foam from ordinary alloys is very difficult because a desiredbubble distribution can not be readily sustained for practical times intheir molten state.

Specifically, the time scales for the flotation of bubbles in a foamscales with viscosity of the material. Accordingly, the mechanicalproperties of these foams drastically degrade with the degree ofimperfection caused by the flotation and bursting of bubbles duringmanufacture. In addition, the low viscosity of most commonly used liquidmetals results in a short time scale for manufacture, which makes theprocessing of metallic foam a delicate process.

In order to remedy these shortcomings, several techniques have beenattempted. For example, to reduce the sedimentation flotation process,Ca particles have been added to the liquid metal. However, the additionof Ca degrades the metallic nature of base metal as well as theresultant metallic foam. Alternatively, foaming experiments have beenperformed under reduced gravity, in space, to reduce the driving forcefor flotation, however, the cost for manufacturing metallic foams inspace is prohibitive.

Accordingly, a need exists for improved methods for manufacturingmetallic foams and especially metallic foams of amorphous atomicstructure which also can be used for the production of better-controlledfoam structures.

SUMMARY OF THE INVENTION

The present invention is directed to a method of controllablymanufacturing metallic foams from amorphous alloys, and moreparticularly to controllably manufacturing metallic foams from bulksolidifying amorphous alloys.

In one embodiment of the invention, the volume fraction of bubbles inthe metallic foam can be continuously varied between >1% and ˜95%. Insuch an embodiment, the bubble size can also be continuously variedbetween ˜2 μm and ˜4 mm on average.

In another embodiment of the invention, the amorphous alloy is abulk-solidifying amorphous alloy, where a bulk-solidifying amorphousalloy is defined as an alloy that can be cast with a dimension of morethan about 1 mm in its smallest dimension.

In another embodiment of the invention, the amorphous alloy is abulk-solidifying amorphous alloy, where a bulk-solidifying amorphousalloy has a delta T of more than 60° C.

In yet another embodiment, the invention is directed to a method ofmaking metallic foams comprising the steps of:

-   -   a) Making a “precursor” by introducing gas bubbles having an        internal “bubble pressure” to the molten alloy at a temperature        above the liquidus temperature of the alloy;    -   b) Cooling the bubble consisting liquid such that it maintains        its amorphous state; and    -   c) Subsequent expansion of the precursor under a havin gradient,        where the pressure during step c must be lower than the bubble        pressure during step a.

In still another embodiment of the invention, the cooling step of themethod entails fully solidifying the precursor into a substantiallyamorphous atomic structure. In such an embodiment, the solidifiedprecursor must be reheated to around the supercooled region in thesubsequent expansion step.

In still yet another embodiment of the invention, the gas bubbles areintroduced to the liquid by stirring the liquid which distributesbubbles through the liquid surface.

In still yet another embodiment of the invention, the gas is introducedto the liquid through a nozzle.

In still yet another embodiment of the invention, the stirring of theliquid is used to chop up existing liquids to obtain smaller bubbles.

In still yet another embodiment of the invention, the gas bubbles areintroduced to the liquid by adding an agent that releases gas at thistemperatures and therefore leads to the creation of bubbles.

In still yet another embodiment of the invention, the method includesthe step of introducing a volume fraction of <30% of small bubbles(between 1 μm and 1 mm) to the molten alloy liquid at or above theliquidus temperature. In such an embodiment, the bubble containingliquid is solidified and its amorphous structure is maintained toproduce a foam “precursor”. In such an embodiment, the foam precursor ispreferably an amorphous metal alloy consisting of up to 30% bubbles witha size distribution between 1 μm and 1 mm.

In still yet another embodiment the invention is directed to a method offorming articles of amorphous metallic foams having a very narrowdistribution of bubble sizes. In such an embodiment the bubbles may havea size distribution of a few gum, for example, between about 1 and 10μm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a graphical representation of the time, temperature andtransformation (TTT diagram) properties of an embodiment(Zr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3) (% atom.) called VIT-106a)of a suitable material for manufacturing metallic foams according to thecurrent invention. A data point showing the time that is available at agiven temperature before crystallization sets in.

FIG. 2 is a graphical representation of the viscosity properties of anembodiment (Zr—Ti—Ni—Cu—Be VIT-1 series) of a suitable material formanufacturing amorphous metallic foams according to the currentinvention.

FIG. 3a is a flowchart of a first embodiment of a method ofmanufacturing amorphous metallic foams according to the currentinvention.

FIG. 3b is a flowchart of a second embodiment of a method ofmanufacturing amorphous metallic foams according to the currentinvention.

FIG. 4a is a graphical representation of the flotation properties of anembodiment (Zr₄₁Ti₁₄Cu₁₂Ni₁₀Be₂₃ (% atom.) called VIT-1) of a suitablematerial for manufacturing amorphous metallic foams according to thecurrent invention

FIG. 4b is a graphical representation of the flotation properties of anembodiment (Zr₄₁Ti₁₄Cu₁₂Ni₁₀Be₂₃ (% atom.) called VIT-1) of a suitablematerial for manufacturing amorphous metallic foams according to thecurrent invention as compared to pure Al metal.

FIG. 5a is a pictorial representation of an embodiment of a solidprecursor manufactured according to the current invention.

FIG. 5b is a pictorial representation of an embodiment of a solidprecursor manufactured according to the current invention.

FIG. 6 is a schematic of an embodiment of an apparatus for manufacturingmetallic foams according to the current invention.

FIG. 7 is a pictorial representation of an embodiment of a solidprecursor (Zr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3) (% atom.) calledVIT-106a) manufactured according to the current invention.

FIG. 8 is a graphical representation of the expansion behavior of theprecursor into a foam at different temperatures(Zr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3) (% atom.) called VIT-106a)of a suitable material for manufacturing metallic foams according to thecurrent invention.

FIG. 9 is a graphical representation of the expansion behavior of thesolid precursor into a foam at different pressures(Zr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3) (% atom.) called VIT-106a)of a suitable material for manufacturing metallic foams according to thecurrent invention.

FIG. 10 is a pictorial representation of an embodiment of an amorphousmetallic foam manufactured according to the current invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method of controllablymanufacturing metallic foams from amorphous alloys, and moreparticularly from bulk-solidifying amorphous alloys.

Bulk solidifying amorphous alloys (or bulk metallic glasses) areamorphous alloys (metallic glass or non-crystalline metal), which can becooled at substantially lower cooling rates, of about 500 K/sec or less,and substantially retain their amorphous atomic structure. As such,these materials can be produced in thicknesses of 1.0 mm or more,substantially thicker than conventional amorphous alloys, which can onlybe formed to thickness of 0.020 mm, and which require cooling rates of10⁵ K/sec or more. Furthermore, bulk-solidifying-amorphous alloysgenerally show a distinct glass transition before crystallization uponheating from the ambient temperatures. Bulk-solidifying amorphous alloysalso generally show a ΔT (defined below) of larger than 30° C.

For the purposes of this invention, the term amorphous means at least50% by volume of the alloy has an amorphous atomic structure, andpreferably at least 90% by volume of the alloy has an amorphous atomicstructure, and most preferably at least 99% by volume of the alloy hasan amorphous atomic structure.

U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975 (thedisclosure of each of which is incorporated herein by reference in itsentirety) disclose such bulk solidifying amorphous alloys. A family ofbulk solidifying amorphous alloys can be described as(Zr,Ti)_(a)(Ni,Cu,Fe)_(b)(Be,Al,Si,B)_(c), where a is in the range offrom 30 to 75, b is in the range of from 5 to 60, and c in the range offrom 0 to 50 in atomic percentages. Furthermore, those alloys canaccommodate substantial amounts of other transition metals up to 20%atomic, and more preferably metals such as Nb, Cr, V, Co. A preferablealloy family is (Zr,Ti)_(a)(Ni,Cu)_(b)(Be)_(c), where a is in the rangeof from 40 to 75, b is in the range of from 5 to 50, and c in the rangeof from 5 to 50 in atomic percentages. Still, a more preferablecomposition is (Zr,Ti)_(a)(Ni,Cu)_(b)(Be)_(c), where a is in the rangeof from 45 to 65, b is in the range of from 7.5 to 35, and c in therange of from 10 to 37.5 in atomic percentages. Another preferable alloyfamily is (Zr)_(a)(Nb,Ti)_(b)(Ni,Cu)_(c) (Al)_(d), where a is in therange of from 45 to 65, b is in the range of from 0 to 10, c is in therange of from 20 to 40 and d in the range of from 7.5 to 15 in atomicpercentages.

Another set of bulk-solidifying amorphous alloys are ferrous metal (Fe,Ni, Co) based compositions. Examples of such compositions are disclosedin U.S. Pat. No. 6,325,868, (A. Inoue et. al., Appl. Phys. Lett., Volume71, p 464 (1997)), (Shen et. al., Mater. Trans., JIM, Volume 42, p 2136(2001)), and Japanese patent application 2000126277 (Publ. #.2001303218A), all of which are incorporated herein by reference. One exemplarycomposition of such alloys is Fe₇₂Al₅Ga₂P₁₁C₆B₄. Another exemplarycomposition of such alloys is Fe₇₂Al₇Zr₁₀Mo₅W₂B₁₅. Although, these alloycompositions are not processable to the degree of the Zr-base alloysystems, they can be still be processed in thicknesses around 1.0 mm ormore, sufficient enough to be utilized in the current invention.

Although any of the above bulk-solidifying amorphous alloys may beutilized, in one preferred embodiment the bulk-solidifying amorphousalloy has a ΔT of larger than 60° C. and preferably larger than 90° C.,where ΔT defines the extent of the supercooled liquid regime above theglass transition temperature, to which the amorphous alloy can be heatedwithout significant crystallization in a typical Differential ScanningCalorimetry experiment.

In general, crystalline precipitates in amorphous alloys are highlydetrimental to their properties, especially to the toughness andstrength, and as such it is generally preferred to limit theseprecipitates to as small a minimum volume fraction possible so that thealloy is substantially amorphous. However, there are cases in which,ductile crystalline phases precipitate in-situ during the processing ofbulk amorphous alloys, which are indeed beneficial to the properties ofbulk amorphous alloys especially to the toughness and ductility. Thevolume fraction of such beneficial (or non-detrimental) crystallineprecipitates in the amorphous alloys can be substantial. Such bulkamorphous alloys comprising such beneficial precipitates are alsoincluded in the current invention. One exemplary case is disclosed in(C. C. Hays et. al, Physical Review Letters, Vol. 84, p 2901, 2000), thedisclosure of which is incorporated herein by reference.

Although bulk-solidifying amorphous alloys are discussed above, itshould be understood that any suitable amorphous alloy, especially oneswith a ΔT of larger than 30° C., may be used in the current invention.

The amorphous alloys and specifically bulk-solidifying amorphous alloysare characterized by relatively sluggish crystallization kinetics. Thesluggish crystallization kinetic makes the whole or a portion of theunder-cooled liquid region, the temperature region between the liquidustemperature and the glass transition temperature, accessible forpractical times, as shown in FIG. 1. For example, the time beforecrystallization sets in was experimentally determined, in an isothermalexperiment, for the whole under-cooled liquid region and is summarizedin time temperature transformation (TTT) diagrams for a few exemplaryamorphous alloys (Zr₄₁Ti₁₄Cu₁₂Ni₁₀Be₂₃,Zr58.5Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3), Pd₄₃Ni₁₀Cu₂₇P₂₀). FIG. 1shows the TTT diagram for Zr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3)(VIT-106a).

The under-cooled region is accessed by cooling from the stable liquid(circles) and by heating the solid amorphous state (squares). At lowtemperatures, below 750 K, no noticeable difference between the heatedand cooled samples in the under-cooled liquid can be observed providedthat such heating and cooling is achieved sufficiently fast to avoid anysignificant crystallization. Furthermore, a relatively large range ofviscosity values can be observed in the under-cooled liquid regime ofbulk-solidifying amorphous alloys. For example, FIG. 2 shows theviscosity as a function of temperature for Zr₄₁Ti₁₄Cu₁₂Ni₁₀Be₂₃ (VIT-1)As shown, the viscosity of this bulk-solidifying amorphous alloy changesby ˜13 orders of magnitude in the undercooled liquid regime.

The applicants discovered that the sluggish crystallization kinetics(see FIG. 1) can be beneficially exploited to develop novel processingmethods for bulk-solidifying amorphous alloy foam structures.Furthermore, the applicants discovered that utilizing these novelprocessing methods and by accessing a large regime of viscosity values,between ˜1 Pa·s and ˜10¹³ Pa·s, highly homogeneous and controllableamorphous metallic foam structures can be obtained. The applicantsfurther discovered that these novel methods of processing amorphousalloys into metallic foam structures can substantially forego or relaxthe dimensional limitations arising from the critical cooling rate toform an amorphous phase.

For example, it is possible to achieve in a bulk solidifying amorphousmetal forming liquid a viscosity of three orders of magnitude higherthan viscosities of pure metals or simple metallic alloys. This highviscosity results in a much slower foaming kinetics. Flotation of thebubbles, coarsening and collapsing scales with the viscosity. Thisshould enable better controllability of factors such as foamhomogeneity, bubbles size distribution, and volume fraction. In thesupercooled liquid region of bulk-solidifying amorphous, a very highviscous state can be achieved where floatation of even centimeter sizebubbles is negligible on the time scale of the experiment.Crystallization is very sluggish wherefore a very controlled expansioncondition of the foam can be established in amorphous metal a techniqueunusable for conventional metallic alloys.

From both a processing point of view and from a materials property viewbulk solidifying amorphous metal are ideal for foam production. Forexample, the high strength of the amorphous alloys is bereficia Nor highstrength to weight foams, and the very high elastic energy absorptioncan be used to make an elastic energy storage foam. The current methodalso makes it possible to produce metallic foams wherein the volumefraction of bubble can be varied almost in a continuous manner to tailorspecific foam properties.

In one exemplary embodiment, the processing method for making foams frombulk-solidifying amorphous alloy exhibiting a glass transition beforecrystallization according to the present invention comprises threegeneral steps: 1) creation of a foam precursor by introducing bubblesinto the liquid form; 2) cooling the precursor; and 3) expanding thebubbles in the precursor to form a final metallic foam. Flow charts oftwo embodiments of this general process are shown in FIGS. 3a and 3b. Asshown, both methods generally entail the steps as recited below.

First, creating a “precursor” at temperatures above the liquidustemperature of the alloy. The “precursor” itself preferably consists ofa moderate volume fraction (<30%) of small bubbles (<1 mm). The methodof forming the precursor preferably including creating a large internalbubble pressure in the bubbles by processing the precursor at highpressures (up to ˜50 bar or more).

Second, the cooling of the precursor from the molten alloy is donesufficiently quickly to avoid any substantial crystallization andmaintain its amorphous state.

Finally, to allow the bubbles in the precursor to expand in thesupercooled liquid region of the bulk-solidifying amorphous alloy undera pressure gradient by processing the material at lower pressures thanthe bubble pressure in step 1 (preferably in partial or full vacuum).The supercooled temperature region is preferably where the viscosity ofthe alloy is between ˜10¹² Pa·s and ˜10⁶ Pa·s. It should be understoodthat the processing time can be any length such that the material doesnot crystallize during expansion or that the process is terminatedbefore crystallization would set in, resulting in an amorphous foam.

In the method summarized in FIG. 3a, the precursor is only cooled in thesecond step to a super-cooled region, shown in the TTT diagram in FIG. 1as being below the nose of crystallization curve and above the glasstransition temperature. Accordingly, in this embodiment, the expansionof the bubbles does not require any reheating of the precursor, butrather controlled cooling of the precursor into specific temperaturezones.

Meanwhile, in the method summarized in FIG. 3b, the precursor is cooledto a solidifying temperature (below the glass transition temperature) inStep 2 to form a solid precursor material, and then reheated in Step 3to above the glass transition temperature to allow for the expansion ofthe bubbles. This embodiment is preferred for manufacturing arrangementsin which it is advantageous to be able to handle a stable precursorprior to the preparation of the final metallic foam.

The expansion of the bubbles, and hence the precursor, can be carriedout in any pre-determined constrained geometry in order to achievenear-to-net-shaped foam components. Furthermore, such operation can becarried as a part of the assembly or mechanical joining operation intoother materials.

Although the process discussed above is useful for a wide variety ofbulk-solidifying amorphous alloys, it should be understood that theprecise processing conditions required for any particularbulk-solidifying amorphous alloy will differ. For example, as discussedabove, a foam consisting of a liquid metal and gas bubbles is anunstable structure, flotation of the lighter gas bubbles due togravitational force takes place, leading to a gradient of the bubbles insize and volume. The flotation velocity of a gas bubble in any liquidmetal material can be calculated according to the Stoke's law:V_(sed)=2a²(ρ_(l)−ρ_(g))g/9η  (1)where g is the gravitational acceleration, a is the bubble radius, andρ_(l), ρ_(g), are the densities of the liquid and gas, respectively.

An exemplary flotation velocity calculation made according to Equation 1for VIT-1 is shown in FIGS. 4a and 4b. As shown in FIG. 4a, usingexperimental viscosity data (as shown in FIG. 2) and a liquid VIT-1density of ρ=6.0×10³ kg/m³, the flotation velocities of bubbles in aVIT-1 alloy melt as a function of bubble radius is calculated for liquidVIT-1 at 950 K (^(———)), and 1100 K (⁻ ⁻ ⁻). FIG. 4b shows the flotationfor a 1 mm gas bubble in liquid VIT-1 (^(———)) and liquid Al (⁻ ⁻ ⁻) asa function of T/T₁.

Using such graphs, acceptable processing conditions, such as time andtemperature can be determined. For example, if the duration of a typicalmanufacturing process is taken to be 60 s and an acceptable flotationdistance of ˜5 mm, processing times and temperatures resulting in aflotation velocity smaller than 10⁻⁴ m/s would be acceptable. Therefore,in this case an unacceptable bubble gradient can be avoided if themaximum bubble size is less than 630 μm if the VIT-1 melt is processedabove its liquidus temperature of about 950 K. By processing VIT-1 meltsat 660 K, below its crystallization temperature of 675 K, no noticeableflotation takes place even for ˜1 cm bubbles. On the other hand, theseresults show that the formation of gradients in Al-melts cannot besuppressed for bubbles larger than about 4 μm.

The TTT-diagram for VIT-1 also suggests that, for example, at ˜700 K ittakes 1100 s before the sample crystallizes. This time is available forprocessing the precursor and expanding the bubbles while avoidingsignificant crystallization. In FIG. 2 the viscosity of VIT-1 isdepicted. In the temperature region where the undercooled liquid isaccessible the viscosity is between 10¹² Pa·s and 10⁶ Pa·s. For theseviscosity values, bubbles of even several cm in size do not show anynoticeable gradient on the time scale of the experiment.

As discussed above, in order to prepare the precursor, a gas has to beintroduced into the liquid bulk-solidifying amorphous alloy. Anysuitable method of introducing bubbles in the liquid bulk-solidifyingamorphous alloy sample may be utilized in the current invention. In oneexemplary embodiment, gas releasing agents, such as B₂O₃ can be usedwhich are mixed with the metal alloy. The B₂O₃ releases H₂O₃ at elevatetemperatures, which in turn forms gas bubbles in the size range of fromabout ˜20 μm up to ˜2 mm. As already demonstrated in the calculations,with these size bubbles no observable gradient takes place in the finalmetallic foam. Exemplary foam materials were made using this gradientfree process, and are shown in FIGS. 5a and 5b for B₂O₃ in a PdNiCuPalloy. These figures also demonstrate how the volume fraction of the gasbubbles can be varied with the processing time, temperature, andpressure between 3% FIG. 5a and 20% FIG. 5b.

Another method to introduce bubbles into a liquid bulk-solidifyingamorphous alloy to obtain a precursor foam is by mechanical treating. Insuch an embodiment, the stability of a liquid surface can be describedby comparing the inertial force to the capillary force, according to theratio:

$\begin{matrix}{W = \frac{\rho\; v^{2}L}{\sigma}} & (2)\end{matrix}$where W is the Weber number, ρ is the density of the liquid, v thevelocity of the moving interface, L a typical length for bubble size,and σ the liquid's surface energy. For W<1 the liquid surface becomesunstable and gives rise to mechanically create bubbles in the liquid.This equation makes it possible to calculate the size of bubbles thatcan be created for a given inertial force and surface energy. Forexample, an object with a velocity of 10 m/s moving in a liquid with adensity of 6.7 g/cm³ and a viscosity of 1 Pa·s is able to break-upbubbles with a size down to 1 μm.

A schematic of an apparatus capable of creating a precursor according tothis method is shown in FIG. 6. In this embodiment, a heated crucible 10holds the liquid alloy sample 12 and a spinning whisk 14 is used tobreakup existing bubbles 16 and create new bubbles 18 by breaking up thesurface 20 of the liquid. A bubbler 22, consisting in this embodiment ofa tube through which gas may be passed is used to create the initialbubbles. Initial bubbles can also be created through the surface by thedrag of the liquid created by the spinning whisk.

An example of a Vitreloy 106 precursor made in accordance with thismechanical method is shown in FIG. 7. The precursor consists of about10% bubbles. The bubble size is in between 0.020 mm and 1 mm.

It should be noted that there is a minimum bubble size that can becreated with the precursor-forming methods. From the energyconsiderations, it can be derived that the minimum bubble size, which isgiven by:Rmin=2Sigma/P  (3)where sigma is the (surface tension) (as in the above Weber equation),and P is the ambient pressure during bubble creation. It should be notedthe bubble size in the foam precursor are preferably as small aspossible in order to obtain a better controlled expansion in thesubsequent steps. According to the above formula, a high ambientpressure (up to 50 bars or more) is desired during bubble formation inorder to create bubbles in smaller diameters.

The invention is directed to methods of achieving a high degree ofhomogeneity in bubble distribution in the foam precursor (which initself can be used a metallic foam material). Nonetheless, the very samefoam precursor can be formed into a final foam material of lower density(a higher volume fraction of bubbles), and with a high degree ofhomogeneity in bubble distribution by utilizing the above-mentionedexpansion steps for the foam precursor with homogeneous bubbledistribution.

In such an embodiment, a first steady-state bubble distribution isachieved with one of the above processes of bubble generation. This isfollowed by flotation of larger bubbles by keeping the molten alloyabove the liquidus. Since large bubbles float much faster than smallbubbles do (see eq. 1) the bubble size distribution can be narrowedsimply by letting the bubbles float. If no new bubbles are generatedduring this step the bubble size distribution shifts towards smallerbubbles and narrows. Accordingly, the specific temperature above theliquidus can be selected by the desire bubble size distribution. Thehigher the temperature above the liquidus, the faster the shift tosmaller bubble sizes and narrowing in the distribution happensFurthermore, after the undesired larger size bubbles are floated, themolten alloy can be homogenized by a controlled mechanical operationwithout trapping additional bubbles, for example by submerging the wholewhisk into the molten alloy. Accordingly, a new bubble distribution canbe achieved with a tighter distribution of smaller bubbles. Theabove-mentioned steps can be repeated several times in order to achievethe desired distribution of bubble size.

Although the viscosity properties of bulk-solidifying amorphous alloysmake it possible to controllably create precursors and prevent seriousspatial gradient in bubble distribution, in conventionalbulk-solidifying amorphous alloy processing techniques it is critical inthe subsequent solidification that the temperature of the foam becontrolled to ensure that substantial crystallization is avoided and theamorphous structure of the material is maintained. As a result, thisrequires cooling the foam material at a rate higher than the criticalcooling rate, where the critical cooling rate, R_(c), is defined as thelowest cooling rate at which significant crystallization of the materialcan be avoided upon cooling. In turn, R_(c) is inversely proportional tothe critical casting thickness, D_(c).

Accordingly, an alloy containing bubbles has a smaller critical castingthickness than the same alloy without any bubbles. Accordingly, theinfluence of the foaming process on the critical casting thickness,assuming the foaming process does not cause heterogeneous nucleation,can be estimated through the increase in thermal diffusion length. Forexample, if α_(g)<<α_(l) (where α is the thermal conductivity of the g(gas), and l (liquid)), ρ_(g)<<ρ_(l), (where ρ is the density), andc_(p,g)≦c_(p,l) (where c_(p) is the specific heat), the heat willpredominately transfer through the liquid. But this requires anincreased diffusion length since the linear path is interrupted.Assuming a dense packing of spherically shaped, uniform bubbles, with avolume fraction of about 75%, the additional diffusion length can becalculated by comparing the length of going around a bubble with thebubble diameter, resulting in a factor of π/2. This results in adecrease in the effective thermal conductivity and gives a criticalcasting thickness for the foam which is 65% of that of the bulkmaterial. Accordingly, amorphous foam containing 75% bubblesmanufactured by this method would be restricted in one dimension to athickness D_(c) (bulk)×0.65.

However, in the technique of the present invention the smallestdimension of the foam is not limited to the Dc of the bulk materials.Specifically, in the first step in the processing route according to thepresent invention an amorphous foam “precursor” consisting of a largenumber of small bubbles (sized between ˜10 μm and ˜1 mm) with a maximumvolume fraction of 30% is formed. The critical casting thickness of theprecursor would be about D_(c) (bulk)×0.8 or larger due to the smallervolume fraction of gas than in the above discussed case with 75%bubbles. This precursor will then subsequently be expanded in thesuper-cooled liquid region. Here, such restrictions of critical castingthickness do not apply. Instead, the dimensions of the final foam islimited by the number and size of the bubbles, the pressure differencein the step 1 and step 3.

In order to expand the bubbles in the precursor in the super-cooledliquid region, a difference in pressure inside the bubbles and thepressure in the undercooled liquid is mandatory. Therefore, thisprocessing step has to be performed at a lower pressure than that usedin Step 1. The expansion time and temperature can be calculated from thegrowth of a gas bubble in a liquid according to Equation 4, below.

$\begin{matrix}{\frac{\mathbb{d}R}{\mathbb{d}t} = {\left( {{P_{B}(R)} - P - \frac{2\sigma}{R}} \right)\frac{R}{4\;\eta}}} & (4)\end{matrix}$where R is the bubble radius, R; interfacial energy, σ, viscosity, η,pressure in the bubble, P_(B); and the pressure outside the bubble, P.

FIG. 8 shows the expanding bubble radius of VIT-106a(Zr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3)% atomic) as a function oftime for different temperatures for a pressure of 3 bar in the bubbleand 10⁻⁶ bar in the liquid. The initial bubble radius is 100 μm. Takenfrom FIG. 1 the time to reach crystallization, which is the availabletime for the foaming process one can calculate the maximum bubble volumefraction for different precursor. This is done for the consideredtemperatures in FIG. 8, namely 700 K, 730 K, 750 K, and 765 K for abubble pressure of 3 bar and a liquid pressure of 10⁻⁶ bar for aninitial bubble radius of 100 μm. These results are also summarized inTable 1, below. For example, a precursor that consists of 10% bubbles,processed at 750 K for the available time of 110 s, expands to a bubblevolume fraction of 47% and maintains its amorphous structure.

TABLE 1 Bubble Expansion Versus Time 5% 10% 20% T [K] T cryst [s] %bubble % bubble % bubble 700 3700 9 18 33 730 420 15 27 45 750 110 30 4767 765 85 33 51 70

FIG. 9 shows the influence of the bubble pressure on the expansion. Theprocessing temperature is 750 K, the initial bubble radius is 100 μm,and the pressure in the liquid during expansion is 10⁻⁶ bar. Table 2shows the expansion of precursors with 5%, 10%, 20% for bubble pressuresof 1 bar, 3 bar, 10 bar, and 30 bar. Especially at high bubble pressurethe precursor can be substantially expanded within the time beforecrystallization sets in.

TABLE 2 Bubble Expansion Versus Pressure 5% 10% 20% T [K] P [bar] %bubble % bubble % bubble 750 1 13 23 41 750 3 30 47 67 750 10 53 71 85750 30 77 88 95

EXAMPLE 1

A low density amorphous PdNiCuP was made by mixing ingots of the PdNiCuPwith hydrated B₂O₃. The B₂O₃ releases gas at temperatures around themelting temperature of the alloy and creates a large number of smallbubbles. The mixture of PdNiCuP and B₂O₃ is processed for 1200 s at 1200K. The bubble containing liquid is then cooled with a rate that preventsdetectable crystallization. The amorphous structure was confirmed bydifferential scanning calorimetry (DSC).

The bubble volume fraction of the precursor is between 10 and 20% (seeFIG. 5a and FIG. 5b). The amorphous precursor was subsequently heated upin the supercooled liquid region to a temperature of 360 C and heldthere for 120 s. The pressure was decreased to about 10⁻³ mbar. Duringthis time the precursor expands. FIG. 10 shows the resulting foam. Thedensity is 2.2×10³ kg/m³ compare to 9.1×10³ kg/m³ of the bulk PdNiCuPsample. This results in a bubble volume fraction of about 75%. DSCmeasurements on the foamed sample showed that no noticeablecrystallization took place during the expansion process.

EXAMPLE 2

Another technique to produce a precursor is to mechanically createbubbles in the liquid by air entrapment. The setup shown in FIG. 6 isused to create the precursor foam. The setup comprises a molybdenumbrush of 3-cm diameter spinning at speeds of up to 2500 rpm. Thisresults in relative velocities between the liquid and brush of up to 3m/s. Small bubbles are then created in the liquid sample which issitting in a graphite crucible that is inductively heated by eitherentrapping gas through the surface, or by releasing gas through abubbler positioned underneath the whisk. In the mechanical airentrapment technique, bubbles are created as a consequence of inducedRayleigh-Taylor instabilities. The Weber number is a dimensionlessscaling number that scales inertia forces to surface tension forces. Itis defined as:we=(density)u²R/sigmawhere u is the relative velocity between liquid and brush and σ is theliquid-gas surface tension. When We>1, inertial forces exceedinterfacial tension forces and consequently an interfacial instabilityis generated by which air gets entrapped in the liquid.

The Weber number can be employed to calculate the size of bubbles thatcan be created by considering that stable bubbles can be formed whenWe>1. Using typical values for density and surface tension as ρ=6500kg/m³ and σ=1 N/m and a relative velocity of 3 m/s, the smallest stablebubble radius that can be created with this parameters is ˜20 microns. AZr_(58.5)Nb_(2.8)Cu_(15.6)Ni_(12.8)Al_(10.3) prefoam synthesized by themechanical air entrapment method is shown in FIG. 7. The prefoamconsists of 10-vol % bubbles with an average size of 250 microns. Thespatial distribution of bubbles appears to be very uniform, whichimplies that sedimentation was negligible during processing. Furthermorethe size distribution of bubbles appears fairly narrow.

The preceding description has been presented with reference to presentlypreferred embodiments of the invention. Workers skilled in the art andtechnology to which this invention pertains will appreciate thatalterations and changes in the described structures and processes may bepracticed without meaningfully departing from the principal, spirit andscope of this invention.

Accordingly, the foregoing description should not be read as pertainingonly to the precise structures described and illustrated in theaccompanying drawings, but rather should be read consistent with and assupport to the following claims which are to have their fullest and fairscope.

What is claimed is:
 1. A method of manufacturing a metallic foam from abulk-solidifying amorphous alloy comprising: providing a moltenbulk-solidifying amorphous alloy; introducing a plurality of gasbubbles, to the molten alloy at a temperature about the liquidustemperature of the alloy to form a precursor at a first pressure suchthat the bubbles are formed with a specified internal bubble pressure;holding the conditions of the precursor after introduction of theplurality of gas bubbles steady for a specified period of time such thata proportion of the plurality of bubbles above a chosen size thresholdare removed from the molten precursor via flotation such that the bubblesize distribution within the precursor is at least partiallyhomogenized; at least partially cooling the precursor to a processingtemperature below the nose of the crystallization curve of the alloy andabove the glass transition temperature of the alloy at a cooling ratesuch that the molten alloy substantially maintains its amorphous state;and expanding the bubbles in the precursor while the precursor is at theprocessing temperature by providing a pressure gradient to the precursorwhere the pressure during the expansion is lower than the internalbubble pressure of the introduced gas bubbles formed during theprecursor forming step.
 2. The method of claim 1, further comprisingquenching the expanded precursor after expanding the bubbles, where thequenching is conducted at a cooling rate such that the at least apartial amorphous atomic structure is formed in the metallic foamobject.
 3. The method according to claim 1, wherein the precursor iscooled to below the glass transition temperature sufficiently fast toform a solidified precursor material with substantially amorphous atomicstructure, and further comprising heating the solid precursor materialinto the super-cooled region of the bulk-solidifying amorphous alloyabove the glass transition temperature of the alloy and below the noseof the crystallization curve of the alloy to expand the bubbles.
 4. Themethod according to claim 1, wherein the temperature of the precursor isreduced to within the supercooled region of the bulk solidifyingamorphous alloy during cooling sufficiently fast to avoid anysubstantial crystallization.
 5. The method according to claim 1, whereinthe gas bubbles are mechanically generated in the molten alloy.
 6. Themethod according to claim 1, wherein the gas bubbles are introduced tothe molten alloy through in gas form through a nozzle.
 7. The methodaccording to claim 1, wherein the gas bubbles are introduced to themolten alloy by adding an a gas releasing agent to the molten alloy. 8.The method according to claim 1, wherein a volume fraction of <30% ofthe plurality of bubbles have sizes between 1 μm and 1 mm.
 9. The methodaccording to claim 1, wherein at least 50% by volume of the metallicfoam has an amorphous atomic structure.
 10. The method according toclaim 1, further including regulating the process parameters during theexpansion in accordance with a calculated size dependent flotationvelocity of the bubbles as given by the equation:V_(sed)=2a²[ρl−ρ_(g)]g/9ρ to control the homogeneity, size and volumedistribution of the bubbles in the precursor.
 11. The method accordingto claim 1, wherein the step of introducing gas bubbles to form theprecursor occurs at a pressure of about 50 bar or more.
 12. The methodaccording to claim 1, wherein the precursor is maintained within atemperature range such that the precursor has a viscosity of about 10⁶Pa·s to 10¹² Pa·s during the expanding step.
 13. The method according toclaim 1, wherein the expansion of the precursor is carried out in one ofeither a mold or a cast.
 14. The method according to claim 1, whereinthe bubbles of the metallic foam have a size distribution of from about1 μm to about 10 μm.
 15. The method according to claim 1, wherein thebulk solidifying amorphous alloy is a Zr-base amorphous alloy.
 16. Themethod according to claim 1, wherein the bulk solidifying amorphousalloy has a ΔT of at least 60° C.
 17. The method according to claim 1,wherein the bulk solidifying amorphous alloy is an Fe-base amorphousalloy.
 18. The method according to claim 1, wherein the plurality ofbubbles is one of either close or open celled.
 19. A method comprising:introducing gas bubbles to an alloy in a molten form at a temperatureabout a liquidus temperature of the alloy at a first pressure to form aprecursor; removing a proportion of the gas bubbles above a chosen sizethreshold such that a size distribution of the bubbles within theprecursor is at least partially homogenized; cooling at least a portionof the precursor such that the alloy substantially maintains anamorphous state; and expanding the bubbles in the precursor to form afoam comprising the alloy substantially in the amorphous state.
 20. Themethod of claim 19, further comprising quenching the expanded precursorafter expanding the bubbles, where the quenching is conducted at acooling rate such that an at least partially amorphous atomic structureis formed in the metallic foam object.
 21. The method according to claim19, wherein the precursor is cooled to below the glass transitiontemperature sufficiently fast to form a solidified precursor materialwith a substantially amorphous atomic structure, and further comprisingheating the solid precursor material into the super-cooled region of thebulk-solidifying amorphous alloy above the glass transition temperatureof the alloy and below the nose of the crystallization curve of thealloy to expand the bubbles.
 22. The method according to claim 19,wherein the gas bubbles are introduced to the molten alloy by adding agas releasing agent to the molten alloy.
 23. The method according toclaim 19, wherein the bubbles have a size distribution of from about 1μm to about 10 μm.
 24. The method according to claim 19, wherein thebulk solidifying amorphous alloy is a Zr-based amorphous alloy or aFe-based amorphous alloy.
 25. The method of claim 19, wherein theprecursor has a pore size distribution of between 1 μm and 10 μm. 26.The method of claim 25, wherein the pore size distribution ishomogeneous.
 27. The method of claim 19, wherein the precursor comprisesless than 30% by volume of gas bubbles below a chosen size thresholdsuch that a size distribution of the bubbles within the precursor issubstantially homogeneous.
 28. The method of claim 19, wherein theprecursor comprises metallic foam.
 29. The method of claim 28, whereinthe metallic foam has a smaller casting thickness than thebulk-solidifying amorphous alloy without any bubbles.